The invention relates generally to reduction of nitrogen oxides (NOx), and particularly, to systems and methods for selective catalytic reduction (SCR) of NOx.
Various methods are used to reduce deleterious effects of air pollution caused by byproducts of high-temperature combustion in internal combustion engines. When combustion occurs in the presence of excess air and at high temperatures, undesirable byproducts, such as nitrogen oxides, commonly known as NOx, are created. NOx emissions are subject to many regulatory provisions limiting the amount of NOx that may be present in effluent gas vented into the surrounding environment.
One known method for dealing with NOx involves the use of selective catalytic reduction (SCR) to reduce NOx to nitrogen gas (N2) using ammonia (NH3) or hydrocarbons as a reductant. However, as ammonia's own hazardous consequences are well known, its use in an SCR system presents additional environmental and other problems that must also be addressed. For example, ammonia slip may result in environmental hazard. As regulatory agencies continue to drive limits on NOx emission lower, other regulations are also driving down the permissible levels of NH3 that may be emitted into the atmosphere.
Because of regulatory limits on ammonia, the use of hydrocarbons and/or their oxygen derivatives for NOx reduction in an SCR process is very attractive. Numerous catalysts have been suggested for this purpose including zeolites, perovskites, and metals on metal oxide catalyst support. However, existing catalyst systems have characteristics, such as low activity, a narrow region of working temperatures, or low stability in the presence of water, that are detrimental to practical use. Other catalyst systems known to reduce NOx have exhibited very little control over the products formed from the reduction and in particular exhibit poor selectivity towards N2 as product.
Therefore there is a need for an effective catalyst system to reduce NOx emissions, which system is stable and operable at a wide range of temperatures.